In general, according to whether lesion is found in the cranial nerve tissue, diseases of the brain are roughly divided into organic brain diseases and functional brain diseases. It is considered that the functional brain diseases among them, in which an organic change (lesion) is not found in cranial nerve tissues, are induced by a certain functional disorder of the brain, unbalance of the cerebral neurotransmitter or an environmental factor or genetic factor.
The number of patients of depression and anxiety disorders as typical functional brain diseases has been increasing every year, and various drugs for their treatment have also been developed. For example, a tricyclic antidepressant, a tetracyclic antidepressant, a monoamine oxidase (MAO) inhibitor, a serotonin and noradrenaline reuptake inhibitor (SNRI), a selective serotonin reuptake inhibitor (SSRI) or the like are used in the treatment of depression. However, problems to be improved are recognized in these drugs, such as their insufficient therapeutic effect, requirement for a prolonged period of time until the onset of their effects, or appearance of side effects including sleepiness, thirst, constipation, difficulty of urination or the like. Additionally, a benzodiazepine, thienodiazepine, non-benzodiazepine drugs or the like are used in the treatment of anxiety disorders. However, problems are also recognized in these drugs, such as insufficient therapeutic effect, or lowering of psychomotor function, lowering of concentration and attentiveness, sleepiness, dizziness, vertigo, headache, amnesia or the like.
On the other hand, it has reported that 2-propylpentanoic acid derivatives including (2R)-2-propyloctanoic acid are useful as a therapeutic and/or preventive agent for neurodegenerative diseases, neural function disorders after stroke or cerebrospinal injury, cerebrospinal diseases accompanied by cerebral tumor or infection or the like since it they have astrocyte function improving activity (e.g., see European Patent No. 0632008 (Patent Document 1).
Also, it has been reported that said 2-propylpentanoic acid derivatives are useful as a therapeutic and/or preventive agent for Parkinson disease or parkinsonian syndrome (e.g., see European Patent Publication No. 1174131 (Patent Document 2)).
Additionally, it is reported also that (2R)-2-propyloctanoic acid is useful as a nerve regeneration accelerator for the treatment and/or prevention of various neurodegenerative diseases (e.g., see International Publication No. 2005/032535 (Patent Document 3)).
However, all of these diseases described in conventionally known references are diseases which accompany organic changes of cranial nerve tissues such as nerve cell degeneration, and so far there is no case reporting that (2R)-2-propyloctanoic acid is effective for a functional brain disease which does not accompany organic changes of cranial nerve tissues.
[Patent Document 1] European Patent No. 0632008
[Patent Document 2] European Patent Publication No. 1174131
[Patent Document 3] International Publication No. 2005/032535